The use of ultrasound to detect fetal anomalies has become standard in the U.S. Although the increase in diagnostic capability has benefited families in some ways, ultrasound and other diagnostic tools have their limitations. The pairing of ultrasound diagnosis with the maternal triple-screen and alpha-fetoprotein tests can significantly increase the validity of the results, but it's possible to see increased nuchal translucency on an ultrasound in a normal fetus.
Gestational Age
Ultrasound diagnosis is most accurate when the fetus is still small but no longer microscopic. Robert Sabbagha, M.D., and his colleagues suggest that "if only one ultrasound examination is to be performed in the second trimester of pregnancy, the best time would be between weeks 19 and 20." This is supported by Anne Frye, CPM, who notes: "Sometimes defects noted at an early exam seem to disappear later"--although one study found this true in one-third of scans supposedly revealing defects.
Technician Skill
Most practitioners emphasize the importance of the ultrasound technician's skill. An accurate image is crucial in determining normal versus abnormal fetal structures. "The ultrasonographer also plays an essential role in the interpretation of the pathophysiology of a discovered defect, and delineating the risk involved. For example, "the risk for spina bifida in a woman with elevated maternal serum alpha-fetoprotein is much lower if a detailed ultrasound study is normal," Sabbagha says.
Maternal History
A woman who is younger than 35 and doesn't have a history of repeat miscarriages and/or stillbirths has a low statistical likelihood of carrying a baby with a genetic syndrome. Age is a huge factor in determining the statistics for fetal anomalies. For example, "The chance of having a baby with Down syndrome gets higher as women get older. If you are less than 30 years old when you have your baby, your chance is less than 1 in 1000. If you are 35 years old, your chance is a bit higher than 1 in 400," says the American College of Nurse-Midwives.
Larger Translucency
At least one study points out an association between greater nuchal translucency and the likelihood of a physical or developmental issue in an infant or young child. O. Adekunle, MRCOG, points out that the "likelihood of a poor pregnancy outcome tends to increase with larger nuchal translucency measure; ranging from 32 percent at 3mm to 100 percent with 5mm" when ultrasound is performed between 10 weeks' and 14 weeks' gestation.
Big Picture
The American College of Nurse-Midwives notes that the nuchal translucency screening done between 10 weeks and 14 weeks' gestation detects approximately 70 percent of fetuses with Down syndrome. This means 30 percent of the other fetuses will likely make it to term, possibly surprising their parents--and possibly giving everyone around them a new perspective on what a normal baby really is.
References
- "Increased nuchal translucency thickness and normal karyotype: time for parental reassurance." Bilardo, C. M.; Müller, M. A.; Pajkrt, E.; Clur, S. A.; van Zalen, M. M.; Bijlsma, E. K Ultrasound in Obstetrics and Gynecology, Volume 30, Number 1, July 2007 , pp. 11-18(8)
- "Ultrasound diagnosis of fetal anomalies." Sabbagha, R, Tamura, R, et al, Glob. libr. women's med., (ISSN: 1756-2228) 2008; DOI 10.3843/GLOWM.10205
- Frye, Anne, CPM. Understanding Diagnostic Tests in the Childbearing Year. Portland, OR: Labrys Press. 1997. pp 707
- "Prenatal Tests for Down Syndrome." American College of Nurse-Midwives, Volume 50, No. 3, May/June 2005.
- "Increased First Trimester Nuchal Translucency: pregnancy and infant outcomes after routine screening for Down syndrome in an unselected antenatal population." Adekunle, O, et al. British Journal of Radiology, 27, 1999. pp 460.
